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516                              CHEMICAL ENGINEERING
it. The lack of data available upon the systems comprising zirconia and the various oxides makes it difficult to make predictions as to the most suitable combinations, but it is evident that there are compounds particularly well suited for specific requirements. In the production of zirconia refractories the chief difficulties lie in the proper molding of the feebly plastic mass and in the large firing shrinkage which can be reduced only by previous calcination at a high temperature. The shrinkage factor is eliminated where it is possible to use ZrSiO4.
Chromite refractories are employed in furnaces as partings between the silica and magnesite bricks and hen,ce are considered neutral. They consist principally of crushed and ground chromite, and bricks are molded from this material which may be bonded together with a small amount of clay or burnt magnesia by pressing under a heavy pressure. The bricks are then dried and fired to a temperature of about cone 16. The composition of these refractories fluctuates between the following values:
PER CENT
Chromium oxide................................  38.0 to 40.0
Alumina.......................................  24.5 to 40.0
Ferric oxide....................................   17.5 to 40.0
Silica..........................................    3.0 to 40.0
Magnesia......................................   15.0 to 40.0
Chrome refractories are excellent conductors of heat. Little information is available concerning their physical properties. They possess a fairly high coefficient of thermal expansion and evidently undergo molecular changes upon reheating. Chrom e brick resist load conditions quite well but are apt to fail suddenly at about l,4oOC. Their use in metallurgical practice has decreased considerably during the past few years. Chrome cement for partings is, however, still being applied.
Fused silica is used as a refractory in pyrometer tubes, crucibles, etc., where its limitation is the change in structure due to crystallization.
Boron and aluminum nitrides are materials suitable for high temperatures but are subject to decomposition and are not used to any extent.
Lime is a basic refractory of the highest type limited in its use through its hydration in the cold state. It requires exceedingly high temperatures for its calcination to constant volume.
The oxides of beryllium, cerium and thorium, etc., are precluded from extensive use owing to their cost. Their employment in gas mantles is too familiar to need further discussion.
Asbestos and other magnesium silicates are sometimes used as refractories. Their low melting point, however, excludes them from high-temperature work.
In addition, many combinations between alumina, magnesia and silica are possible which may render good service under special conditions. As an example, a composition suitable for spark plugs and high-tension insulators used at about red heat, as in the Cottrell process, is as follows:
PER CENT
Calcine:
Kaolin............................................        75.10
Magnesite.........................................         8.77
Alumina..........................................        16.13
Body:
Calcine...........................................        60.00
Kaolin............................................        30.00
Ball clay..........................................        10.00